Systems and methods for treating carbon dioxide

ABSTRACT

A heat exchange assembly for treating carbon dioxide (CO 2 ) is described. The heat exchange assembly includes a housing that includes an inlet, an outlet, and an inner surface that defines a cavity extending between the inlet and the outlet. The housing is configured to receive solid CO 2  through the inlet. At least one heat exchange tube extends through the housing. The heat exchange tube is oriented to contact solid CO 2  to facilitate transferring heat from solid CO 2  to a heat exchanger fluid being channeled through the heat exchange tube to facilitate converting at least a portion of solid CO 2  into liquid CO 2 . The heat exchange assembly is configured to recover a refrigeration value from the solid CO 2  and transfer at least a portion of the recovered refrigeration value to a flue gas.

This Application is a Division of patent application Ser. No.13/285,375, filed on Oct. 31, 2011, the contents of which areincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

This invention was made with Government support under Contract No.DE-AR0000101, awarded by the Department of Energy. The Government hascertain rights in this invention.

BACKGROUND OF THE INVENTION

The subject matter described herein relates generally to gas treatmentsystems and, more particularly, to gas treatment system for use intreating carbon dioxide (CO₂).

At least some known power generation systems include a combustor and/orboiler to generate steam that is used in a steam turbine generator.During a typical combustion process within a combustor or boiler, forexample, a flow of combustion gases, or flue gases, is produced. Knowncombustion gases contain combustion products including, but not limitedto, carbon, fly ash, carbon dioxide, carbon monoxide, water, hydrogen,nitrogen, sulfur, chlorine, arsenic, selenium, and/or mercury.

At least some known power generation systems include a gas treatmentsystem for use in reducing an amount of combustion products within theflue gases. Known gas treatment systems include a low-temperaturecooling system for separating CO₂ from the flue gases. During operation,the low-temperature cooling system cools a flue gas stream to form solidCO₂ from gaseous CO₂ suspended within the flue gas stream. In addition,at least some known gas treatment systems include a low-temperaturesolids pump for use in transporting the solid CO₂ from thelow-temperature cooling system to a CO₂ sequestration system forsequestration and deposition of the CO₂. During operation, thelow-temperature cooling system transfers a refrigeration value to theflue gas stream to form solid CO₂. As the low-temperature solids pumpconveys the solid CO₂ from the cooling system, at least some of therefrigeration value may be lost to heat generated from operation of thesolids pump. The loss of refrigeration value through the solids pumpincreases the cost of operating the gas treatment system by increasingan amount of energy required to cool the flue gas stream.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, a heat exchange assembly for treating carbon dioxide(CO₂) is provided. The heat exchange assembly includes a housing thatincludes an inlet, an outlet, and an inner surface that defines a cavityextending between the inlet and the outlet. The housing is configured toreceive solid CO₂ through the inlet. At least one heat exchange tubeextends through the housing. The heat exchange tube is oriented tocontact solid CO₂ to facilitate transferring heat from solid CO₂ to aheat exchanger fluid being channeled through the heat exchange tube tofacilitate converting at least a portion of solid CO₂ into liquid CO₂.The heat exchange assembly is configured to recover a refrigerationvalue from the solid CO₂ and transfer at least a portion of therecovered refrigeration value to a flue gas.

In another aspect, a gas treatment system for use in treating carbondioxide (CO₂) in a flue gas is provided. The gas treatment systemincludes a cooling system coupled to a source of flue gas and configuredto receive a flow of flue gas from the source. The cooling system isconfigured to cool gaseous CO₂ suspended within the flue gas to formsolid CO₂. A heat exchange assembly is coupled to the cooling system forreceiving a flow of solid CO₂ from the cooling system. The heat exchangeassembly is configured to recover a refrigeration value from the solidCO₂ and transfer at least a portion of the recovered refrigeration valueto the flue gas. The heat exchange assembly includes a housing thatincludes an inlet, an outlet, and an inner surface that defines a cavityextending between the inlet and the outlet. The housing is configured toreceive solid CO₂ through the inlet. At least one heat exchange tubeextends through the housing. The heat exchange tube is oriented tocontact solid CO₂ to facilitate transferring heat from solid CO₂ to aheat exchanger fluid being channeled through the heat exchange tube tofacilitate converting at least a portion of solid CO₂ into liquid CO₂.

In yet another aspect, a method of treating carbon dioxide (CO₂) isprovided. The method includes channeling a flue gas containing CO₂ to acooling system to cool the flue gas to form solid CO₂, and channelingsolid CO₂ to a heat exchanger assembly. The heat exchanger assemblyincludes a housing that is configured to receive solid CO₂ therein, andat least one heat exchange tube extending through the housing. Apressure within the housing is adjusted to maintain the housing pressurewithin a predefined range of pressures to prevent re-sublimation ofsolid CO₂. A flow of heat exchange fluid is channeled through the atleast one heat exchange tube to facilitate a transfer of heat from solidCO₂ to the heat exchange fluid to convert at least a portion of solidCO₂ into liquid CO₂, and to recover a refrigeration value from the CO₂.At least a portion of the recovered refrigeration value is transferredto the flue gas to facilitate cooling the flue gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an exemplary power generation system.

FIG. 2 is a schematic view of an exemplary heat exchanger assembly thatmay be used with the power generation system shown in FIG. 1.

FIGS. 3-4 are schematic views of alternative embodiments of the heatexchanger assembly shown in FIG. 2.

FIG. 5 is an alternative embodiment of the power generation system shownin FIG. 1.

FIG. 6 is another embodiment of the power generation system shown inFIG. 1.

FIG. 7 is a flow chart of an exemplary method that may be used to treatcarbon dioxide generated during operation of the power generation systemshown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The exemplary systems and methods described herein overcome at leastsome disadvantages of known gas treatment systems by providing a gastreatment system that includes a heat exchange assembly that isconfigured to transfer heat from a heat exchange fluid to solid CO₂ tofacilitate recovering a refrigeration value from solid CO₂. Moreover,the heat exchange assembly is configured to maintain CO₂ in solid-liquidphase equilibrium to enable the heat exchange assembly to transfer heatto solid CO₂ to facilitate forming liquid CO₂ for use in pre-cooling aflue gas. By providing a gas treatment system that includes a heatexchange assembly configured to recover a refrigeration value from solidCO₂, the cost of treating CO₂ suspended within a flue gas is reduced ascompared to known gas treatment systems.

FIG. 1 is a schematic view of an exemplary power generation system 10.FIG. 2 is a schematic view of an exemplary heat exchange assembly 12that may be used with power generation system 10. In the exemplaryembodiment, power generation system 10 includes a combustor assembly 14,a steam generation assembly 16 downstream of combustor assembly 14, anda steam turbine engine 20 coupled to steam generation assembly 16.Combustor assembly 14 includes at least one combustor 22, a fuel supplysystem 24, and an air supply system 26. Fuel supply system 24 is coupledto combustor 22 for channeling a flow of fuel such as, for example, coalto combustor 22. Alternatively, fuel supply system 24 may channel anyother suitable fuel, including but not limited to, oil, natural gas,biomass, waste, and/or any other fossil and/or renewable fuel thatenables power generation system 10 to function as described herein. Inaddition, air supply system 26 is coupled to combustor 22 for channelinga flow of air to combustor 22. Combustor 22 is configured to receive apredetermined quantity of fuel and air from fuel supply system 24 andair supply system 26, respectively, and ignite the fuel/air mixture togenerate combustion or flue gases. Moreover, combustor 22 channels aflow of flue gases 28 to steam generation assembly 16 to generate steamthat is channeled to steam turbine engine 20 for use in generating apower load.

In the exemplary embodiment, steam generation assembly 16 includes atleast one heat recovery steam generator (HRSG) 30 that is coupled inflow communication with a boiler feedwater assembly 32. HRSG 30 receivesa flow of boiler feedwater 33 from boiler feedwater assembly 32 tofacilitate heating boiler feedwater 33 to generate steam. HRSG 30 alsoreceives flue gases 28 from combustor assembly 14 to further heat boilerfeedwater 33 to generate steam. HRSG 30 is configured to facilitatetransferring heat from flue gases 28 to boiler feedwater 33 to generatesteam, and channel steam 34 to steam turbine engine 20. Steam turbineengine 20 includes one or more steam turbines 36 that are rotatablycoupled to a power generator 38 with a drive shaft 40. HRSG 30discharges steam 34 towards steam turbine 36 wherein thermal energy inthe steam is converted to mechanical rotational energy. Steam 34 impartsrotational energy to steam turbine 36 and to drive shaft 40, whichsubsequently drives power generator 38 to facilitate generating a powerload.

In the exemplary embodiment, power generation system 10 includes a gastreatment system 42 that is downstream from combustor assembly 14 andsteam generation assembly 16. Gas treatment system 42 is configured toreceive flue gases 28 exhausted from combustor assembly 14 and/or steamgeneration assembly 16 to facilitate removing combustion productsincluding, but not limited to, carbon, fly ash, carbon dioxide, carbonmonoxide, water, hydrogen, nitrogen, sulfur, chlorine, arsenic,selenium, and/or mercury from the flue gases.

In the exemplary embodiment, gas treatment system 42 includes a flue gaspre-cooling system 44, a low-temperature cooling system 46 downstream offlue gas pre-cooling system 44, a CO₂ separator 48 downstream of coolingsystem 46, heat exchange assembly 12 downstream of cooling system 46,and a CO₂ utilization system 52 downstream of heat exchange assembly 12.Flue gases 28 including gaseous CO₂ and nitrogen (N₂) are channeled toflue gas pre-cooling system 44 from combustor assembly 14 and/or steamgeneration assembly 16. Flue gas pre-cooling system 44 facilitates aheat transfer from flue gases 28 to a heat exchange fluid 54 beingchanneled through flue gas pre-cooling system 44 to facilitate reducinga temperature of flue gases 28. Pre-cooling system 44 channels thecooled flue gases 28 to cooling system 46.

Cooling system 46 is configured to treat flue gases 28 to cool gaseousCO₂ within flue gases 28 to form solid CO₂. Cooling system 46 channelscooled flue gases 28 and solid CO₂ to CO₂ separator 48 to facilitateseparating solid CO₂ and N₂ from flue gases 28. CO₂ separator 48channels solid CO₂ 56 to heat exchange assembly 12 to facilitatetransferring heat from heat exchange fluid 54 to solid CO₂ 56. Moreover,heat exchange assembly 12 is configured to facilitate transferring ofheat from solid CO₂ 56 to heat exchange fluid 54 being channeled throughheat exchange assembly 12 to facilitate converting solid CO₂ 56 toliquid CO₂ 58. Moreover, heat exchange assembly 12 is configured torecover a refrigeration value from solid CO₂ 56 and transfer at least aportion of the recovered refrigeration value to the flue gases 28 tofacilitate cooling flue gases 28. In addition, heat exchange assembly 12is configured to channel liquid CO₂ 58 to CO₂ utilization system 52 forutilization of rich CO₂. In one embodiment, CO₂ utilization system 52includes a sequestration system for sequestration of rich CO₂.Alternatively, utilization system 52 may include any system configuredto use CO₂ for any purpose. In the exemplary embodiment, heat exchangeassembly 12 is also configured to adjust a temperature and a pressurewithin heat exchange assembly 12 such that CO₂ within heat exchangeassembly 12 is in solid-liquid phase equilibrium.

Heat exchange assembly 12 includes a heat exchanger 60 and a lockhopperassembly 62. Lockhopper assembly 62 is coupled between heat exchanger 60and CO₂ separator 48 for channeling solid CO₂ 56 from CO₂ separator 48to heat exchanger 60. Lockhopper assembly 62 includes a tank 64 that isconfigured to receive solid CO₂ 56 from CO₂ separator 48, and a valveassembly 66 coupled between tank 64 and heat exchanger 60 to enablesolid CO₂ 56 to be selectively channeled to heat exchanger 60 from tank64. Lockhopper assembly 62 is configured to adjust a pressure withintank 64 such that a pressure within tank 64 is within a range ofpressures such that solid CO₂ 56 remains in the solid phase. Inaddition, lockhopper assembly 62 is configured to enable solid CO₂ 56 tobe gravity fed from tank 64 into heat exchanger 60. In the exemplaryembodiment, lockhopper assembly 62 is configured to maintain an interiorpressure equal to about 7 atm.

Heat exchanger 60 includes a housing 68 and at least one heat exchangetube 70 that extends through housing 68. Housing 68 includes an inlet72, an outlet 74, and an inner surface 76 that defines a cavity 78extending between inlet 72 and outlet 74. Housing 68 is configured tomaintain a pressure within cavity 78 within a predefined range ofpressures to facilitate preventing re-sublimation of solid CO₂ 56 togaseous CO₂ within cavity 78. In the exemplary embodiment, housing 68 isconfigured to maintain an internal pressure of about 7 atm. In oneembodiment, housing 68 is configured to maintain an internal pressurebetween about 1 atm to about 10 atm. Moreover, lockhopper assembly 62channels a flow of pressurized fluid to housing cavity 78 through valveassembly 66 to pressurize housing 68 to a predefined pressure. In theexemplary embodiment, inner surface 76 includes an upper portion 80, anda lower portion 82 that extends below upper portion 80. Inlet 72 extendsthrough upper portion 80 and is coupled to lockhopper assembly 62 forreceiving solid CO₂ 56 from lockhopper assembly 62. In addition, housinglower portion 82 is sized and shaped to contain liquid CO₂ 58 therein.Outlet 74 extends through lower portion 82, and is coupled to CO₂utilization system 52. More specifically, heat exchange assembly 12includes a liquid CO₂ pump 84 that is coupled between heat exchanger 60and CO₂ utilization system 52 for channeling liquid CO₂ 58 from lowerportion 82 to CO₂ utilization system 52.

In the exemplary embodiment, heat exchange tube 70 extends thoughhousing cavity 78, and is configured to channel a flow of heat exchangefluid 54 through housing cavity 78. Heat exchange tube 70 extends alonga centerline axis 85 between a first end 86, and a second end 88. Heatexchange tube 70 is oriented within cavity 78 such that an outer surface90 of heat exchange tube 70 contacts solid CO₂ 56 to facilitatetransferring heat from heat exchange fluid 54 to solid CO₂ 56 toincrease a temperature of solid CO₂ 56 and facilitate converting atleast a portion of solid CO₂ 56 to liquid CO₂ 58.

Heat exchange assembly 12 also includes a plurality of fins 92 thatextend outwardly from tube outer surface 90. Each fin 92 includes anouter surface 94 that is configured to contact solid CO₂ 56 tofacilitate transferring heat from heat exchange fluid 54 to solid CO₂ 56to facilitate forming liquid CO₂ 58 from solid CO₂ 56, and to cool heatexchange fluid 54 to recover a refrigeration value from solid CO₂ 56.Each fin 92 is oriented within cavity 78 such that solid CO₂ 56 is atleast partially supported by heat exchange tube 70 within housing upperportion 80. Moreover, each fin 92 is oriented to channel liquid CO₂ 58formed within cavity 78 from upper portion 80 to lower portion 82 suchthat liquid CO₂ 58 is collected within a pool 96 formed within lowerportion 82. In the exemplary embodiment, each fin 92 is orientedsubstantially perpendicular to centerline axis 85. In addition, each fin92 is at least partially submerged within liquid CO₂ 58 to facilitatetransferring heat from liquid CO₂ 58 to heat exchange fluid 54. In oneembodiment, heat exchange tube 70 includes a plurality of pipes 98 thatare each coupled to one or more fins 92. Each pipe 98 is oriented withincavity 78, and extends between first end 86 and second end 88. One ormore pipes 98 are at least partially submerged within liquid CO₂ 58 tofacilitate transferring heat from liquid CO₂ 58 to heat exchange fluid54.

During operation of system 10, combustor 22 receives a predefinedquantity of fuel from fuel supply system 24, and receives a predefinedquantity of air from air supply system 26. Combustor 22 injects the fuelinto the air flow, ignites the fuel-air mixture to expand the fuel-airmixture through combustion, and generates high temperature flue gases.Combustor 22 channels flue gases 28 to HRSG 30 to facilitate generatingsteam from flue gases 28. In addition, boiler feedwater assembly 32channels a flow of boiler feedwater 33 to HRSG 30. HRSG 30 transfersheat from flue gases 28 to boiler feedwater 33 to facilitate heatingboiler feedwater 33 to generate steam 34. HRSG 30 discharges steam 34towards steam turbine 36 wherein thermal energy in the steam isconverted to mechanical rotational energy. HRSG 30 and/or combustor 22discharge flue gases 28 toward gas treatment system 42 to facilitatetreating carbon dioxide CO₂ suspended within flue gases 28.

In the exemplary embodiment, HRSG 30 and/or combustor 22 channel fluegases to pre-cooling system 44. Pre-cooling system 44 transfers heatfrom flue gases 28 to heat exchange fluid 54 to reduce a temperature offlue gases 28. Pre-cooling system 44 discharges flue gases 28 towardscooling system 46 to facilitate generating solid CO₂ from gaseous CO₂suspended within flue gases 28. In addition, pre-cooling system 44channels heat exchange fluid 54 towards heat exchange assembly 12.Cooling system 46 cools flue gases 28 to generate solid CO₂ and channelscooled flue gases 28 and solid CO₂ 56 to CO₂ separator 48 to facilitateseparating solid CO₂ and N₂ from flue gases 28. CO₂ separator 48discharges solid CO₂ towards lockhopper assembly 62. In addition, CO₂separator 48 channels a flow of CO₂ lean gas 100 that includes a mixtureof CO₂ and N₂ to cooling system 46 and/or lockhopper assembly 62. In oneembodiment, CO₂ lean gas 100 discharged from CO₂ separator 48 is dividedinto a first sub-stream 102 and a second sub-stream 104. Firstsub-stream 102 is discharged to atmosphere. Second sub-stream 104 iscompressed in a compressor 106 and channeled to lockhopper assembly 62at a predefined pressure to facilitate adjusting a pressure withinlockhopper assembly 62.

Lockhopper assembly 62 channels solid CO₂ 56 towards heat exchanger 60to transfer heat from solid CO₂ 56 to heat exchange fluid 54 beingchanneled through heat exchanger 60. Solid CO₂ 56 is gravity fed to heatexchanger 60 to transfer heat from heat exchange fluid 54 to solid CO₂56 to convert at least of portion of solid CO₂ 58 to liquid CO₂ 58, andto cool heat exchange fluid 54 to recover a refrigeration value fromsolid CO₂ 56. Heat exchanger 60 discharges liquid CO₂ 58 to CO₂utilization system 52. In addition, heat exchanger 60 channels heatexchange fluid 54 towards pre-cooling system 44 for use in cooling fluegases 28.

In the exemplary embodiment lockhopper assembly 62 and heat exchanger 60each include an internal pressure equal to about 7 atm to facilitatepreventing re-sublimation of solid CO₂ 56 to gaseous CO₂ within cavity78, and to maintain CO₂ in solid-liquid phase equilibrium. Lockhopperassembly 62 channels solid CO₂ 56 having a temperature equal to about−102° C. towards heat exchanger 60. Heat exchange fluid 54 is channeledinto heat exchanger 60 includes a temperature equal to about −51° C. Assolid CO₂ 56 contacts of heat exchange tube 70, at least a portion ofsolid CO₂ 56 is converted to liquid CO₂ 58. Liquid CO₂ 58 dischargedfrom heat exchanger 60 includes a fluid temperature equal to about −51°C. Heat exchange fluid 54 discharged from heat exchanger 60 includes afluid temperature equal to about −80° C.

FIGS. 3-4 are schematic views of alternative embodiments of heatexchange assembly 12. Identical components shown in FIGS. 3-4 arelabeled with the same reference numbers used in FIG. 2. In analternative embodiment, heat exchange tube 70 extends between a firstsection 108 and a second section 110. First section 108 is orientedwithin lower portion 82 such that first section 108 is at leastpartially submerged within liquid CO₂ 58. Second section 110 is orientedwithin upper portion 80, and is configured to support solid CO₂ 56 suchthat solid CO₂ 56 is oriented above liquid CO₂ pool 96. Fins 92 arecoupled to heat exchange tube 70 and are oriented obliquely with respectto centerline axis 85 to facilitate channeling liquid CO₂ 58 from upperportion 80 to lower portion 82. One or more fins 92 are coupled to tubefirst section 108, and are at least partially submerged within liquidCO₂ 58.

Referring to FIG. 4, in another embodiment, each fin 92 is coupled tosecond section 110 of heat exchange tube 70 such that each fin 92 isoriented within upper portion 80. Each fin 92 is oriented with respectto an adjacent fin 92 such that a plurality of openings 112 are definedbetween adjacent fins 92. Each opening 112 is sized and shaped tochannel liquid CO₂ 58 from upper portion 80 to lower portion 82.

FIG. 5 is another embodiment of power generation system 10. Identicalcomponents shown in FIG. 5 are labeled with the same reference numbersused in FIG. 1. In an alternative embodiment, heat exchanger 60 channelscold liquid CO₂ 58 to flue gas pre-cooling system 44 for use inpre-cooling flue gases 28. More specifically, liquid CO₂ pump 84channels liquid CO₂ 58 from heat exchanger 60 to flue gas pre-coolingsystem 44. In addition, flue gas pre-cooling system 44 channels liquidCO₂ 58 to CO₂ utilization system 52. In one embodiment, liquid CO₂ pump84 is configured to channel liquid CO₂ 58 through flue gas pre-coolingsystem 44, and to CO₂ utilization system 52.

FIG. 6 is an alternative embodiment of power generation system 10.Identical components shown in FIG. 6 are labeled with the same referencenumbers used in FIG. 1. In an alternative embodiment, power generationsystem 10 includes a top cycle or gas turbine engine assembly 114 and abottom cycle or steam turbine engine 20. Gas turbine engine assembly 114includes a compressor 116, a combustor 118 downstream of compressor 116,and a turbine 120 downstream of combustor 118 and powered by gasesdischarged from combustor 118. Turbine 120 drives an electricalgenerator 122. In addition, turbine 120 discharges flue gases 28 to HRSG30 for generating steam from flue gases 28.

In the exemplary embodiment, heat exchanger 60 is coupled downstream ofpre-cooling system 44 for receiving a flow of flue gases 28 frompre-cooling system 44. During operation HRSG 30 and/or turbine 120discharge flue gases 28 to pre-cooling system 44 to transfer heat fromflue gases 28 to liquid CO₂ 58. Pre-cooling system 44 channels fluegases 28 to heat exchanger 60 to transfer heat from flue gases 28 tosolid CO₂ 56 to form liquid CO₂ 58 from solid CO₂ 56 to facilitatecooling flue gases 28, and to recover a refrigeration value from solidCO₂ 56. Heat exchanger 60 channels cooled flue gases 28 to coolingsystem 46 to cool flue gases 28 to form solid CO₂ from gaseous CO₂suspended within flue gases 28. Cooling system 46 channels solid CO₂ andflue gases 28 to CO₂ separator 48 to separate solid CO₂ from flue gases28, and discharge solid CO₂ to lockhopper assembly 62. Lockhopperassembly 62 discharges solid CO₂ 56 to heat exchanger 60 to transferheat from solid CO₂ 56 to flue gases 28 being channeled through heatexchanger 60, and to form liquid CO₂ 58 from solid CO₂ 56. Heatexchanger 60 channels liquid CO₂ 58 to pre-cooling system 44 tofacilitate transferring heat from flue gases 28 to liquid CO₂ 58. Inaddition, pre-cooling system 44 channels liquid CO₂ 58 to CO₂utilization system 52.

FIG. 7 is a flow chart of an exemplary method 200 that may be used totreat CO₂ that is generated during an operation of power generationsystem 10. In the exemplary embodiment, method 200 includes channeling202 solid CO₂ from lockhopper assembly 62 to heat exchange assembly 12,and channeling 204 a flow of heat exchange fluid 54 through heatexchange tube 70 to facilitate a transfer of heat from solid CO₂ to heatexchange fluid 54 to form liquid CO₂ from solid CO₂, and to recover arefrigeration value from solid CO₂ and liquid CO₂. Method 200 alsoincludes adjusting 206 a pressure within housing 68 to maintain ahousing pressure within a predefined range of pressures to preventre-sublimation of solid CO₂ to gaseous CO₂. Heat exchange fluid 54 ischanneled 208 from heat exchange assembly 12 to pre-cooling system 44 topre-cool flue gases 28. Liquid CO₂ is channeled 210 from heat exchangeassembly 12 to a CO₂ utilization system 52 to facilitate utilization ofrich CO₂.

The above-described systems and methods overcome at least somedisadvantages of known gas treatment systems by providing a gastreatment system that includes a heat exchange assembly configured totransfer heat from a heat exchange fluid to solid CO₂ to facilitaterecovering a refrigeration value from solid CO₂. In addition, the gastreatment system includes a heat exchange assembly that is configured tomaintain CO₂ in solid-liquid phase equilibrium to enable the heatexchange assembly to transfer heat from solid CO₂ to a heat exchangefluid to facilitate forming liquid CO₂ for use in pre-cooling a fluegas. By providing a gas treatment system that includes a heat exchangeassembly that recovers a refrigeration value from solid CO₂, the cost oftreating CO₂ suspended within a flue gas is reduced as compared to knowngas treatment systems.

Exemplary embodiments of systems and methods for treating carbon dioxideare described above in detail. The systems and methods are not limitedto the specific embodiments described herein, but rather, components ofsystems and/or steps of the method may be utilized independently andseparately from other components and/or steps described herein. Forexample, the systems and method may also be used in combination withother gas treatment systems and methods, and are not limited to practicewith only the gas treatment system as described herein. Rather, theexemplary embodiment can be implemented and utilized in connection withmany other gas treatment system applications.

Although specific features of various embodiments of the invention maybe shown in some drawings and not in others, this is for convenienceonly. In accordance with the principles of the invention, any feature ofa drawing may be referenced and/or claimed in combination with anyfeature of any other drawing.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal language of the claims.

What is claimed is:
 1. A gas treatment system for use in treating carbondioxide (CO₂) in a flue gas, said gas treatment system comprising: acooling system coupled to a source of flue gas and configured to receivea flow of flue gas from the source, said cooling system configured tocool CO₂ within the flue gas to form solid CO₂; and a heat exchangeassembly coupled to said cooling system for receiving a flow of solidCO₂ from said cooling system, wherein said heat exchange assembly isconfigured to recover a refrigeration value from the solid CO₂ andtransfer at least a portion of the recovered refrigeration value to theflue gas, said heat exchange assembly comprising: a housing comprisingan inlet, an outlet, and an inner surface that defines a cavityextending between said inlet and said outlet, said housing configured toreceive solid CO₂ through said inlet; and at least one heat exchangetube extending through said housing, said heat exchange tube oriented tocontact solid CO₂ to facilitate transferring of heat from solid CO₂ to aheat exchange fluid being channeled through said heat exchange tube tofacilitate converting at least a portion of solid CO₂ into liquid CO₂.2. A gas treatment system in accordance with claim 1, wherein saidhousing is configured to maintain said cavity within a predefined rangeof pressures to prevent re-sublimation of solid CO₂ to gaseous CO₂.
 3. Agas treatment system in accordance with claim 1, wherein said housinginner surface extends between an upper portion and a lower portionextending below said upper portion, said lower portion configured tocontain liquid CO₂ therein.
 4. A gas treatment system in accordance withclaim 3, wherein said at least one tube includes an outer surface and aplurality of fins extending outwardly from said tube outer surface, eachfin of said plurality of fins is configured to support solid CO₂ withinsaid upper portion.stf
 5. A gas treatment system in accordance withclaim 4, wherein each fin of said plurality of fins is at leastpartially submerged within liquid CO₂.
 6. A gas treatment system inaccordance with claim 4, wherein adjacent fins are oriented such that aplurality of openings are defined between adjacent fins, each opening issized to channel liquid CO₂ from said upper portion to said lowerportion.
 7. A gas treatment system in accordance with claim 1, furthercomprising a lockhopper assembly coupled between said cooling system andsaid heat exchange assembly for receiving solid CO₂ from said coolingsystem, said lockhopper assembly configured to enable solid CO₂ to begravity fed into said housing cavity.
 8. A gas treatment system inaccordance with claim 1, further comprising a CO₂ sequestration systemcoupled to said heat exchange assembly for receiving a flow of liquidCO₂ from said heat exchange assembly.
 9. A method of treating carbondioxide (CO₂), said method comprising: channeling a flue gas containingCO₂ to a cooling system to cool the flue gas to form solid CO₂;channeling the solid CO₂ to a heat exchange assembly, wherein the heatexchange assembly includes a housing configured to receive solid CO₂therein, and at least one heat exchange tube extending through thehousing; adjusting a pressure within the housing to maintain the housingpressure within a predefined range of pressures to preventre-sublimation of solid CO₂; channeling a flow of heat exchange fluidthrough the at least one heat exchange tube to facilitate a transfer ofheat from solid CO₂ to the heat exchange fluid to convert at least aportion of solid CO₂ into liquid CO₂ and to recover a refrigerationvalue from the CO₂; and transferring at least a portion of the recoveredrefrigeration value to the flue gas to facilitate cooling the flue gas.10. A method in accordance with claim 9, further comprising channelingthe heat exchange fluid to a pre-cooling system to facilitatetransferring the recovered refrigeration value to the flue gas tofacilitate cooling the flue gas.
 11. A method in accordance with claim9, further comprising channeling solid CO₂ from a lockhopper assembly tothe heat exchange assembly, wherein the lockhopper assembly isconfigured to enable solid CO₂ to be gravity fed into the housingcavity.
 12. A method in accordance with claim 9, further comprisingchanneling liquid CO₂ from the heat exchange assembly to a CO₂sequestration system.